Investigations on the nature of the phytochrome-induced transmitter for the regulation of nitrate reductase in etiolated leaves of maize

A lag phase of 30 min for phytochrome-mediated stimulation in the induction of nitrate reductase (NR) in etiolated leaves of Zea mays var. 'Ganga 5', was eliminated by a 2 h pre-irradiation with red light, supplied in the absence of nitrate. Earlier, photoreversibility of NR activity was found to be lost completely by 2 h and a transmitter with a life span of 12 h was proposed to mediate the phytochrome effect on NR (Sharma and Sopory, 1984). Low temperature (0°C) treatment prevented the loss of the photoreversibility. Almost complete photoreversibility was observed after 2 h, suggesting that the formation of the transmitter requires active metabolism. Tungstate, given 2 h after red light treatment, inhibited the increase in the enzyme activity by red light, suggesting that the transmitter is not inactive NR itself, which becomes activated by nitrate. Analysis of steady-state levels of the NR transcript revealed that NR mRNA is not induced in response to Pfr formation in the absence of nitrate treatment, suggesting that it could not be the transmitter mediating the phytochrome effect on NR activity. Red light was found to increase the uptake of 45Ca2+ by isolated maize protoplasts. When supplied exogen-ously calcium increased NR activity by 48% of that obtained by red light irradiation. However, exogenous addition of phorbol myristate acetate (PMA), an activator of protein kinase C, was found to increase the NR transcript level to the same extent obtained after 5 min of red light irradiation. The results suggest that phytochrome may be acting through calcium along with messengers like diacylglycerol, generated through the phosphoinositide (PI) cycle to stimulate the induction of NR